Total knee arthroplasty involves the replacement of portions of the patellar, femur and tibia with artificial components. In particular, a proximal portion of the tibia and a distal portion of the femur are cut away (resected) and replaced with artificial components.
As used herein, when referring to bones or other body parts, the term “proximal” means closest to the heart and the term “distal” means more distant from the heart. When referring to tools and instruments, the term “proximal” means closest to the practitioner and the term “distal” means distant from the practitioner. However, when a tool or instrument is fixated to a bone or other body part the terms “proximal” and “distal” are applied to the tool or instrument as if the tool or instrument were itself a bone or body part.
There are several types of knee pros theses known in the art. One type is sometimes referred to as a “resurfacing type.” In these pros theses, the articular surface of the distal femur and proximal tibia are “resurfaced” with respective metal and plastic condylar-type articular bearing components.
The femoral component is typically a metallic alloy construction (e.g. cobalt-chrome alloy or 6A4V titanium alloy) and provides medial and lateral condylar bearing surfaces of multi-radii design of similar shape and geometry as the natural distal femur or femoral-side of the knee joint.
One important aspect of these procedures is the correct resection of the distal femur and proximal tibia. These resections must provide planes which are correctly oriented in order to properly accept the prosthetic components. Among the factors that are considered when assessing resection of the distal femur and proximal tibia are the proximal-distal location of the resection planes, the varus-valgus angle of the planes, and the change in relative orientation of the planes in response to change in flexion-extension angle of the knee.
Moreover, following distal resection the femur is shaped with the aid of a cutting block. To ensure correct shaping of the femur, the cutting block must be correctly positioned and sized. More specifically, the cutting block must be correctly positioned with respect to the anterior-posterior direction and must be correctly rotated about an axis perpendicular to the distal resection plane such that the block's rotation corresponds to the correct Internal/External (I/E) rotation of the femur relative to the tibia. The I/E rotation may be set in a number of ways. One way of setting I/E rotation is by referencing the angle formed between the cutting block's medial-lateral axis as projected onto the distal resection plane and the knee's posterior condylar axis as projected onto the distal resection plane. In a typical case, the angle formed between the cutting block's medial-lateral axis as projected onto the distal resection plane and the knee's posterior condylar axis as projected onto the distal resection plane is set to approximately 3 degrees and matches the angle formed between the epicondylar axis as projected onto the distal resection plane and the posterior condylar axis as projected onto the distal resection plane.
In addition, the cutting block should be correctly positioned with respect to the medial-lateral direction. However, medal-lateral positioning of the block is not critical to the femur shaping procedure and, as such, does not require the same degree of precision as exercised during anterior-posterior positioning of the block and I/E rotation of the block.
A typical cutting block includes two or more fixation pegs, or “pins” that are used for positioning the block on the distal resection plane and securing the block to the plane. In practice, the block to be used is known and thus the positions of the pins within the block are known. Therefore, one can set the block's position in space by setting the pins' position in space. Accordingly, to position the block on the distal plane the appropriate pin positions are determined, pinholes are drilled at the determined positions, the pins in the block are lined up with the pinholes, and the pins are inserted into the pinholes to secure the block to the femur.
In many cases, the appropriate cutting block and the correct pinhole positions are determined using an instrument referred to as an “Anterior-Posterior Sizer” (or “AP Sizer”). The Sizer is designed to determine the appropriate cutting block and correct pinhole positions based on the type and size of femoral component that will be implanted. For example, the implant could be from the line of implants associated with the Stryker® Triathlon® Knee System which includes femoral implants of sizes 1-8. In such context, the AP Sizer will determine the size of Triathlon® implant that is needed and will indicate where the pinholes should be located for a cutting block corresponding to the Triathlon® implant of the determined size.
However, implants from different manufacturers or different lines may differ. Accordingly the cutting blocks and associated hole positions needed for each type of implant may differ, and therefore the AP Sizers required for each type of implant may differ. For this reason, a large number of different AP Sizers is typically required to support a large number of implant-types.
Further, there are many different types of methodologies employed for determining the correct implant size and hole position. For example, implant size and hole position can be determined through use of a “mechanical stylus,” a “navigation stylus,” a “blade runner,” or “drill sizing.” The type of sizing used in a procedure is often left to the discretion of the practitioner, with most practitioners having a preference for one method over the others.
For purposes of clarity of presentation, the various types of methodologies employed for determining the correct implant size and hole position will hereinafter be collectively referred to as “sizing methodologies.”
Despite the many different sizing methodologies that may be employed, AP Sizers are typically designed for only one methodology. Thus, if a practitioner would like to change methodologies, the practitioner would have to change sizers. For example, a practitioner who prefers blade runner sizing may find that blade runner sizing is inaccurate on a particular patient due to patient specific conditions such as taught muscle tone or localized trauma to the desired referencing zone. In such event, the practitioner may wish to size through navigation rather than blade running. However, to make the change from blade runner sizing to navigation sizing, the practitioner would need to change from a sizer designed for blade runner sizing to a sizer designed for navigation sizing.
In short, prior AP Sizers have been implant-specific and methodology-specific, and therefore large numbers of AP Sizers have been required to support the various implant types and sizing methodologies.